Protected/permissive left-turn (PPLT) traffic controls increase the left-turn capacity and reduce delay at intersections by providing an exclusive turn phase for left turns as well as a phase during which left turns can be made as opposing traffic will allow. The protected left turn can either lead (or precede) or lag (or follow) the opposing through signal phase. PPLT controls have been implemented in a variety of ways, because the Manual on Uniform Traffic Control Devices (MUTCD) provides limited guidance. At least six displays to indicate the permissive phase are known to exist in the United States (i.e., straight five-section head with circular green, five-section cluster head with circular green, flashing circular red, flashing circular yellow, flashing red arrow, and flashing yellow arrow). Variations also exist in the phasing, signal displays, arrangement, signal placement, and use of supplemental signs. There have been concerns that some of these variations may confuse motorists, and validation of their relative operational and safety advantages was needed. A key concern with PPLT control is the "yellow trap," which occurs during the change from permitted left turns in both directions to a lagging protected left turn in one direction. The MUTCD requires that all circular signal indications on an approach to an intersection display the same color. The left-turning driver whose permitted interval is ending may try to sneak through the intersection on the yellow indication, not realizing that the opposing through traffic still has a green indication. To avoid the yellow trap, most agencies do not use leading/lagging PPLT. An innovation known as "Dallas Display" allows this operation without the yellow trap by operating the permissive left turns simultaneously with the opposing through movement. Previous research has shown that this operation reduces delay and improves safety, but is not easily implemented in all situations.